Tuesday, 30 April 2013

When we adults are confronted by a bridge, we're concerned not just by its width and sturdiness, but also by the height of the drop beneath. If there's a deep canyon, we'd usually rather the bridge was mighty strong and wide. If there's but a short drop, we'll happily jaunt along the narrowest, flimsiest of crossings - after all, it won't matter much if we fall.

Infants - those aged 11 to 14 months - are different. They don't want to fall, so they're wary of narrow bridges. But the height of the drop makes no difference to them at all. "We found clear evidence that infants are averse to falling from a height," said the researchers Kari Kretch and Karen Adolph, "but no evidence of adult-like anxiety that increases with drop-off height."

Kretch and Adolph challenged 37 14-month-olds to walk across a bridge of varying widths spanning a 76cm gap between two surfaces. The drop beneath the bridge was either large (71cm - nearly the infants' standing height) or short (17cm - roughly knee-high to the infants). An experimenter was on-hand to prevent any falls.

When faced with a more narrow crossing, the toddlers were more cautious as you'd expect - they hesitated, felt their way, and proceeded more slowly. Too narrow and they'd even refuse to go ahead. Crucially, however, their crossing behaviour didn't vary according to the height of the drop. A similar result was found when the study was repeated with 11-month-olds who were still crawling.

It's not that the walkers and crawlers couldn't perceive the difference in the height of the drops. When they refused to cross a very narrow bridge, they'd climb down into the small drop, but not the big drop.

At first, these new results might appear to contradict Gibson and Walk's classic "visual cliff" experiments conducted in the 1960s, in which babies refused to crawl onto a glass surface that had the appearance of a cliff edge. However, the visual cliff studies, and other research since, didn't disentangle risk of falling from the issue of fall height and the likelihood of injury. The researchers point out their new results aren't as surprising as they might seem. Toddlers are effectively averse to all dangers of falling, whether down a short or big drop. Unlike adults, they don't calibrate according to the relative risk.

"How would infants know that the longer an object (or baby) falls, the harder it hits the ground?" asked Kretch and Adolph. "Certainly by adulthood, we understand this intuitively. An open question is how and when this understanding develops." They acknowledged it would be useful for future research to explore a broader range of heights, to see if there's any level at which toddlers do register a greater danger.

_________________________________
Kretch, K., and Adolph, K. (2013). No bridge too high: Infants decide whether to cross based on the probability of falling not the severity of the potential fall. Developmental Science, 16 (3), 336-351 DOI: 10.1111/desc.12045